The amino acid histidine, on the basis of a number of short term studies, is considered to be dispensable for adult man, the adult male chicken and, under certain conditions, the adult rat. Histidine is a precursor of the two dipeptides, anserine and carnosine which are found in considerable quantity in muscle tissue, while carnosine has recently been reported to occur also in the brain, where it may serve as a possible neurotransmitter for olfactory sensations. The purpose of the study herein proposed is to evaluate the need for a dietary source of histidine by the adult chicken and rat in relation to the distribution of anserine and carnosine in muscle and brain tissue as a function of age and of the histidine concentration in the diet. Furthermore, it is proposed to study directly whether the level of carnosine in the brain (olfactory bulb) transmits olfactory sensations. Adult rats on either histidine containing or histidine-free diets will be tested as to their ability to discern differences in smell. Finally, it is proposed to study the relationship between dietary histidine deprivation and the ability of the adult rat and male chicken to maintain nitrogen equilibrium, including the maintenance of reasonable concentrations of histidine derivatives. Through the use of carbon 14 labeled compounds an attempt will be made to measure the ability of adult chickens to synthesize histidine from precursors which are known to serve in this capacity in microorganisms.